lkml.org 
[lkml]   [2020]   [Oct]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH] printk: handle blank console arguments passed in.
    From
    Date
    On 10/6/20 2:52 AM, Petr Mladek wrote:
    > On Mon 2020-10-05 20:35:59, Guenter Roeck wrote:
    >> On 10/5/20 7:59 PM, Sergey Senozhatsky wrote:
    >>> On (20/05/22 12:00), Petr Mladek wrote:
    >>>> On Fri 2020-05-22 16:53:06, Shreyas Joshi wrote:
    >>>>> If uboot passes a blank string to console_setup then it results in a trashed memory.
    >>>>> Ultimately, the kernel crashes during freeing up the memory. This fix checks if there
    >>>>> is a blank parameter being passed to console_setup from uboot.
    >>>>> In case it detects that the console parameter is blank then
    >>>>> it doesn't setup the serial device and it gracefully exits.
    >>>>>
    >>> Petr, this patch's causing regressions for us. We use blank console= boot
    >>> param to bypass dts. It appears that it'd be better to revert the change.
    >>>
    >> Not just to bypass dts, it was also possible to use console= to disable consoles
    >> passed as config option, as well as other default console options. A quick test
    >> confirms that this affects all platforms/architectures, not just Chromebooks.
    >> Prior to this patch, it was possible to disable a default console with an
    >> empty "console=" parameter. This is no longer possible. This means that
    >> this patch results in a substantial (and, as far as I can see, completely
    >> undiscussed) functionality change.
    >
    > Where is this behavior documented, please?
    >

    I don't know. I didn't find it either. All I know is that Chromebooks
    apparently used it from day 1 to disable the console, and I always thought
    it was official behavior until I stumbled over the problem last weekend,
    tried to look it up, and failed to find it.

    > I do not see it anywhere (documentation, git log, google) and it is far from
    > obvious from the code. It seems that any random string would do the
    > same job, e.g. console=none.
    >

    Agreed about the "far from obvious". From looking at the code, it seems like
    an unintended (?) side effect to me.

    > Of course, we need to restore the original behavior when it breaks
    > existing systems. But I want to be sure that there is no better
    > solution.
    >
    > And it makes perfect sense to disable all consoles or drop all defined
    > by dts. But I would prefer to make it more obvious way, for
    > example by parameters like:
    >
    > + console=none
    > + no-console
    > + no-dtd-console
    > + no-default-console
    >
    Again, the problem isn't limited to dts provided consoles, or at least
    that was my understanding. I am still trying to understand how default
    consoles are defined, so I may get something wrong. Anyway, personally I
    liked "console=", but that is just me. Anything else should work for us
    as long as it is backward compatible (which excludes the no-xxx options).

    Whatever is decided, I'd like to have it made official and documented to
    avoid a similar problem in the future.

    >
    > JFYI, the console= parameter handling is a real historical mess. We are
    > always surprised what undefined behavior people depend on. For
    > example, see:
    >
    > + commit 33225d7b0ac9903c5701b ("printk: Correctly set CON_CONSDEV
    > even when preferred console was not registered")
    >
    > + commit e369d8227fd211be3624 ("printk: Fix preferred console
    > selection with multiple matches")
    >
    >> I don't understand why (yet), but the patch also causes regressions with
    >> seemingly unrelated functionality, specifically with dm-verity on at least
    >> one Chromebook platform. I filed crbug.com/1135157 to track the problem,
    >> and reverted the patch from all our stable releases immediately after
    >> the last round of stable release merges.
    >>
    >> On a side note, I don't see the problem presumably fixed with this
    >> patch in any of my tests.
    >
    > Console drivers might provide a custom match() callback to handle
    > various aliases. I guess that some driver wrongly matches the empty
    > string stored in the array of preferred consoles.
    >

    That might well be. Obviously all Chromebooks never had a problem with it.
    I'll keep trying; maybe I can find a qemu emulation that crashes with it.

    Unfortunately we don't have a traceback, so it is difficult to determine
    what actually caused the problem. Maybe Sergey can provide one.

    > There are likely other ways to fix the original problem.
    >
    Most definitely. Either case, again, I'd like to make sure that we get
    some official means to disable a pre-configured console using the
    command lime.

    Thanks,
    Guenter

    \
     
     \ /
      Last update: 2020-10-06 12:46    [W:2.624 / U:0.024 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site